1. Field of the Invention
The present invention relates to a process for working up a liquid reaction discharge of the cationic polymerization of isobutene, which substantially comprises polyisobutene, unconverted isobutene and an inert diluent.
2. Description of the Background
High molecular weight polyisobutenes having molecular weights of up to several 100 000 Dalton have long been known and their preparation is described, for example, in H. Güterbock: Polyisobutylen und Mischpolymerisate, pages 77–104, Springer, Berlin 1959. The highly reactive polyisobutenes which have a high content of terminal vinylidene groups, preferably substantially more than 60 mol %, should be distinguished from these conventional polyisobutenes. Highly reactive polyisobutenes are desirable intermediates for the preparation of additives for lubricants and fuels.
Such highly reactive polyisobutenes are obtainable, for example, by the process of EP 0 628 575, by cationic polymerization of isobutene in the liquid phase with the aid of boron trifluoride and a secondary alcohol at from 0 to −60° C.
The prior patent applications DE 199 48 947.5, DE 199 52 031.3, DE 199 52 030.5, DE 100 28 585.6 and DE 100 35 298.7 relate to improvements or advantageous embodiments of such a process.
Owing to the comparatively high viscosity of polyisobutene at room temperature and below, the polymerization of isobutene is carried out as a rule in the presence of an inert diluent. The diluent used serves in particular for reducing the increase in the viscosity of the reaction mixture which is observable during the polymerization reaction, to such an extent that the resulting heat of reaction can be removed sufficiently rapidly. After the desired molecular weight has been reached, the reaction is stopped by deactivating the polymerization catalyst. Preferably, the polymerization catalyst or its deactivation products is or are removed by extraction with an aqueous solution.
The polyisobutene formed is then freed from the unconverted isobutene and the diluent, which is usually effected by distillation, first the readily volatile isobutene and then the diluent having a higher boiling point than isobutene being distilled off. The polyisobutene remains as a distillation residue. The thermal load during the distillation is however problematic because residues of deactivation products of the polymerization catalyst, for example traces of hydrogen fluoride, can, on heating, lead to undesired secondary reactions of the unconverted isobutene or subsequent reactions of the polyisobutene formed. One undesired subsequent reaction is, for example, the acid-catalyzed isomerization of a polyisobutene molecule having a terminal vinylidene double bond to a polyisobutene molecule in which the double bond occupies a thermodynamically more favorable position in the interior of the molecule. Undesired secondary reactions of the unconverted isobutene are, for example, the dimerization to isooctene or the formation of tert-butanol from isobutene and traces of water contained in the reaction mixture.